This invention relates to a powder feeding apparatus for conveying powder through a duct by means of a gas, a high-performance electrostatic powder coating apparatus obtained through application of this powder feeding apparatus, and an improved powder flow-rate measuring apparatus for use in these apparatuses.
FIGS. 20 and 22 show a conventionally known automatic control system for feeding powder by means of a gas. This system is used to correctly supply a relatively small amount, e.g., several tens to several hundred grams per minute, of an expensive powder material, to each of several to several tens of apparatuses, as in the case of powder supply to powder coating apparatuses, thermal spraying apparatuses or the like.
Referring to FIG. 20, the length and diameter of a measurement duct 101 and the diameter of a nozzle 103n provided at the inlet of the measurement duct 101, are determined in such a way that a negative pressure which is generated by an injector effect due to a combination of the nozzle 103n and the measurement duct 101 when a measurement gas 118, whose flow rate is maintained at a fixed value by a flow-rate control means 102, is blown into the measurement duct 101 through the nozzle 103n, and a positive pressure generated by the gas measurement gas 118 when it flows through the measurement duct 101, are substantially nullified by cancelling each other. By thus determining the length and diameter of the measurement duct 101 and the diameter of the nozzle 103n, it is always possible to maintain the pressure difference between the inlet 104 and the outlet 105 of the measurement duct 101 at a fixed value of not more than several mm Hg when the velocity of the measurement gas 118 flowing through the measurement duct 101 is approximately in the range of 5 to 30 m/sec.
Under these conditions, when a fluidization gas 116 is dispersed, as indicated by arrows 117, through a porous plate 115 at the bottom of a powder tank 113, powder in the tank 113 is fluidized and introduced to the inlet 104 of the measurement duct 101. Then, the powder is accelerated by the measurement gas as it flows through the measurement duct 101, thereby generating a pressure difference in proportion to the mass flow rate of the introduced powder between the inlet 104 and the outlet 105. Conversely, by measuring this pressure difference, it is possible to measure the flow rate of the powder passing through the measurement duct 101.
In FIG. 20, the pressure at the inlet 104 is introduced to a high-pressure chamber 126 of a differential pressure gauge 106 through a capillary 107 and a connecting tube 122, whereas the pressure at the outlet 105 is introduced to a low-pressure chamber 125 of the differential pressure gauge 106 through a capillary 108 and a connecting tube 120. The differential pressure gauge 106 includes a pressure plate 123, which is supported by a flexible ring 124 constituting the partition of this differential pressure gauge. The pressure difference between the two chambers of the differential pressure gauge 106 acts on the pressure plate 123 to cause a displacement thereof, which displacement is converted into a differential pressure signal 128 in the form of a pneumatic signal, electric signal or the like by a conversion mechanism 127, and is conveyed through a signal processing device 129 which performs signal processing, such as amplification, as needed. The signal is then input to a control device 131.
The control device 131 compares a set value 132 with the input signal, indicated at 130 (i.e., the output from the signal processing device 129), and amplifies the difference thereby obtained. The output of the control device 131, indicated at 133, is used to operate a control valve 134 so as to control the flow rate of compressed gas 136, which is introduced to a nozzle 137 of an injector 139, which consists of the nozzle 137 and a throat 138, through a pipe 135. The flow rate of the compressed gas 136 is controlled so as to adjust the negative pressure at the measurement duct outlet 105, which constitutes the vacuum chamber of the injector 139, in such a way that the pressure difference between the inlet 104 and the outlet 105 of the measurement duct 101, that is, the mass flow rate of the powder, is constantly matched with the set value 132 of the control device 131, thereby constantly maintaining the mass flow rate of the powder in a gas/powder two-phase flow 141, supplied through a feeding duct 140, at a predetermined value.
If the flow rate of the gas supplied through the nozzle 137 is not high enough, the velocity of the gas flowing through the feeding duct 140 is rather low, resulting in pulsations being generated in the powder feed. Such pulsations can be prevented by providing the injector outlet with a gas inlet 149 for an auxiliary carrier gas 150 so that the proper feeding rate can be ensured.
In order that the gas pressures at the inlet 104 and the outlet 105 of the measurement duct 101 may be detected and communicated without involving a reverse flow of the powder, fixed amounts of purge gases 110 and 112, whose flow rates are correctly controlled by flow rate control means 109 and 111, respectively, are introduced into the capillaries 107 and 108 through tubes 121 and 119, respectively, in such a way that the gas flow velocities in the capillaries 107 and 108 are maintained at fixed values of not less than 15 m/sec.
A working curve of this apparatus is obtained in the following manner: an air permeable sack for collecting powder is fitted onto the outlet end section of the feeding duct 140 in order to measure the amount of powder fed in a fixed period of time. From this measured value, the amount of powder fed per unit time is calculated. By performing calculations in this way, a first working curve 153 as shown in FIG. 23 is obtained with respect to this particular system. In FIG. 23, the x-axis indicates the amount of powder fed per unit time, and the y-axis indicates differential gauge output as displayed on a display device 130i.
The first working curve 153 obtained in this way is generally hard to use as it is, since it involves variations in intercept and inclination due to various factors, such as the machining precision for the system components 101, 103n, 107, 108, 119, 120, 121, 122, 106, etc., the conditions for the assembly of the components, the installation and piping of the system, and the physical properties of the powder. In particular, such variations constitute a problem when a plurality of powder feeding apparatuses are operated in parallel since that requires the respective working curves of the different systems to be equalized.
It should be noted in this regard that the purge gas 110 is capable of shifting the working curve in the positive direction along the y-axis through a pressure drop in the capillary 107, that the purge gas 112 is capable of shifting the working curve in the negative direction along the y-axis through a pressure drop in the capillary 108, and that the measurement gas 118 is in a positive relation mainly to the inclination of the working curve. Thus, by adjusting the respective flow rates of these gases, the working curve can be modified. For example, by increasing the purge gas 110 by an empirically known amount, it is possible to modify the first working curve 153 of FIG. 23 in such a way that its intercept is shifted to zero, as in the case of a working curve 154 shown in FIG. 24.
Further, by increasing the measurement gas 118 by an empirically known amount, the inclination of the working curve can be enhanced as in a working curve 155 shown in FIG. 24. In this way, it is possible to adjust the working curve to a predetermined inclination, which, in the case of FIG. 24, is one at which the differential pressure gauge output is 200 mmAg when the amount of powder fed per unit time is 200 g/min.
In these adjustments, the points in the small circles in FIGS. 23 and 24 cannot be located without performing the collection and measurement of powder at least two times for each of these points, which means a considerable amount of time and labor is required for these adjustments.
In the case of a powder coating apparatus or the like, an intermittent supply of powder is required. For this purpose, a powder valve 148 is provided, which, as shown in FIG. 20, consists of a pinch rubber member 143 fitted into the interior of a housing 142 provided between the measurement duct outlet 105 and the injector 139.
The opening and closing of the powder valve 148 is effected, for example, in the following manner: pressure as indicated by an arrow 147 is applied to the outer periphery of the pinch rubber member 143 by a three-way valve 146 through a pipe 145. This causes an inward deformation of the pinch rubber member 143, which is then brought to a condition as indicated at 143' in FIG. 21. As a result, the communication between the injector 139 and the measurement duct outlet 105 is disconnected, thereby stopping the powder supply.
At this time, the injector 139 is usually also stopped by stopping the supply of the compressed gas 136 for driving the injector, by means of an electromagnetic valve or the like (not shown). In this process, the purge gases 110 and 112 and the measurement gas 118 are generally allowed to continue to flow so that a reverse flow or intrusion of powder is prevented. In this condition, the powder is caused to flow back to the powder tank 113.
When re-starting the powder supply, the pinch rubber member 143 is released from the above-mentioned pressure by the three-way valve 146 to restore it to the former condition as indicated at 143 by virtue of its elasticity, etc. At the same time, the supply of the compressed gas 136 for driving the injector is started.
FIG. 22 shows the essential part of another means for preventing powder from entering the ducts for communicating the pressures at the inlet 104 and the outlet 105 of the measurement duct 101 to the differential pressure gauge 106. Apart from this essential part, the structure of this means is the same as that shown in FIG. 21. In the case of the structure shown in FIG. 22, the pressures at the inlet 104 and the outlet 105 are respectively transmitted to the differential pressure gauge 106 through porous plates 151 and 152, the capillaries 107 and 108, and the connecting tubes 120 and 122. In this structure, it is necessary to provide purge gases 110 and 112 for the purpose of preventing changes in gas-flow resistance due to clogging of the porous plates and appropriately adjusting the intercept of the working curve.
Regarding the means for detecting the pressure at the inlet 104 of the measurement duct, it is also possible to prevent intrusion of powder by opening the capillary 107 at a position which is near the outlet of the nozzle 103n and in the upstream thereof, or to arrange the capillary 107 and the porous plate 151 at other positions which are at the same level as the inlet 104 of the duct for measuring the fluidized powder in the tank.
Apart from the conventional techniques described above, various other means are in use. For example, a powder feeding means is available, in which the powder in the feeding tank is adjusted in various ways so that the filling factor of the powder is kept from being influenced by the powder level in the feeding tank so as to hold the filling factor constant and, in this condition, the powder is extracted by extracting means, such as pore-row raking-out means, groove raking-out means, or drawing means using a precision screw feeder, before the conveying means using a gas is applied.
The operation of these means, however, have to be stopped from time to time so as to perform actual quantity measurement. Generally speaking, under the existing circumstances, it can be said that a means for measuring and controlling powder mass flow has not been known yet which operates accurately, which is inexpensive, which has a simple structure, whose interior can be easily cleaned, which excels in stability for long-term use, and which can be used in combination with a gas carrying system.
Apart from the above, a "loss-in-weight" system is known, according to which the weight of the powder hopper and that of the extraction means are constantly measured in their entirety; the results are differentiated to calculate a value corresponding to the instant feeding amount; and the extraction means is automatically controlled in such a way that the above value is kept at a fixed value. This system, however, has a problem in that separation of the hopper has to be effected for each of the extraction means, each extraction means requiring a measuring device to which powder must be supplied, resulting in the entire apparatus becoming very complicated and expensive. Thus, the range of applications for this system is very limited.
In the above-described conventional automatic control systems (hereinafter referred to as the "prior-art techniques", shown in FIGS. 21, 22, 23 and 24, the interior of the measurement duct 101 is usually made of a non-adhesive resin, such as fluororesin or high-density polyethylene. Despite such a material, some powder may be deposited on the inner surface of the measurement duct, depending upon the properties of the powder and those of the measurement gas, with the result that the configuration of the inner surface of the duct is changed, thereby making it impossible to accurately measure.
There is constantly a gas flow of 15 to 20 m/sec or more in each of the capillaries 107 and 108 for detecting and communicating the pressure difference generated between the inlet 104 and the outlet 105 of the measurement duct 101 in proportion to the powder flow rate. Despite this gas flow, some electrically charged powder can flow up into the capillaries 107 and 108 due to the pressure fluctuations inevitably generated in the gas flow or the gas/powder two-phase flow in the measurement duct or some other place, or due to the variations or fluctuations in pressure caused by the repeated operations of the pinch rubber member. The powder flowing up into the capillaries 107 and 108 will stick to the inner surfaces of these capillaries to cause the flow resistance thereof to change and, further, flow up into the tubes 119 and 121 and the connecting tubes 120 and 122, sticking to the inner surfaces thereof. Clumps of this sticking powder may be separated by mechanical shock or the like and thereafter clog the capillaries 107 and 108. To prevent this, these tubes 107, 108, 119, 121, 122, etc. have to be periodically cleaned, which requires a considerable amount of cost and labor.
The generation of errors and malfunctions caused by powder flowing up into these tubes due to pressure fluctuations, etc. is more liable to occur in proportion to the interior volumes of the high-pressure-side tubes 121 and 122, the interior volume of the high-pressure-side chamber 126 of the differential pressure gauge, the interior volumes of the low-pressure-side tubes 119 and 120, the interior volume of the low-pressure-side chamber 125 of the differential pressure gauge, and the degree of displacement of the pressure plate 123 and the partition 124 of the differential pressure gauge. Further, the powder flowing up into these tubes may enter the differential pressure gauge, causing malfunctions thereof.
In the structure shown in FIG. 22, in which differential pressure is communicated through the porous plates 151 and 152, instead of the capillaries 107 and 108, in order to avoid reverse flow of powder, the purge gases flowing through these porous plates cause clogging with the passage of time due to a minute reverse flow caused by pressure fluctuations even though the purge gases flow generally in the directions indicated by arrows 110 and 112. As a result, the pressure drop in the porous plates gradually increases, thereby making it impossible to prevent generation of large errors.
This is attributable to the compression of the gas upstream of the porous plates, fluctuations in volume, and deformation of the piping, and is, consequently, inevitable. The above condition is also due to adhesion and solidification of electrically charged powder on the porous plates. It is impossible to avoid such phenomenons even by using porous plates of finer mesh. On the contrary, use of such porous plates of finer mesh would lead to an increase in the pressure drop of the purge gases 110 and 112 as they pass through the porous plates 151 and 152, which pressure drop, together with the clogging caused by a trace quantity of particles inevitably contained in the purge gases 110 and 112, would cause malfunction of the differential pressure detection system.
The four tubes 119, 120, 121 and 122 used for the measurement of differential pressure in the conventional example shown in FIGS. 21 and 22, are rather numerous, so that they will be obstructive during field work, such as during the change of colors of powder coating materials. These numerous tubes will also lead to a considerable cost for the piping. Further, the flow rate control means 109 and 111 require combined use of an automatic constant-pressure valve, an infinitesimal-flow-rate regulating valve, an infinitesimal-flow meter, etc, resulting in an expensive system. Moreover, the installation and adjustment of the system must be conducted scrupulously, resulting in a lot of time being required.
Further, to conduct field work, such as color change, the piping of the system should not be made of inflexible materials such as metal or hard plastic; it is necessary to employ piping consisting of flexible hoses, with the result that the length, configuration, etc. of the piping and the characteristics of the differential pressure detection system differ from unit to unit. This also leads to bothersome installation and adjustment operations.
In the example shown in FIGS. 21 and 22, the purge gases 110 and 112 are eventually united with the powder carrier gas and thereby increase the feeding rate. As a result, the performance of the injector 139 is impaired, and the amount of the compressed gas 136 is increased. This is particularly undesirable when the total amount of carrier gas has to be as small as possible to maintain a gentle discharge pattern at the piping end, as is often the case with electrostatic powder coating.
Further, the purge gas 110 on the upstream side has an influence on the acceleration of the powder in the measurement duct 101, so that, however precise the dimensions of the measurement duct 101 may be, it is not possible to constantly maintain the differential-pressure-generation characteristics, which depend upon acceleration, with the result that the inclinations and intercepts of the working curves shown in FIGS. 23 and 24 interfere with each other. Thus, a lot of time and expense is required for adjustments and measurements, resulting in high costs.
Further, even when no powder is being supplied, the purge gases 110 and 112 and the measurement gas 118 must continue to flow, resulting in an increase in cost, scattering of powder, shifts in grain size distribution, etc, which should not be overlooked.
In the conventional techniques described with reference to FIGS. 21, 22, etc., the fluctuations in the powder feed amount in the case, for example, of a powder coating apparatus, are mainly attributable to the following four factors: a fluctuation in the level of the coating material contained in the tank 113; a reduction of the inner diameter of the feeding duct 140 due to adhesion of powder to the inner surface thereof; a degeneration in performance due to wear of the injector throat 138; and a change in the gun level when coating a long and large object which is vertically suspended. The amount of the compressed gas 136 increases or decreases according to the above four factors, making it possible to automatically adjust the powder feed amount to a predetermined value.
The change in feeding rate caused in this process is usually approximately 5 to 15%, which may be acceptable for practical uses. However, there is an increasing demand for an expansion of the range of applications of powder coating and for a more exact quality control regarding film thickness and coating efficiency. From this viewpoint, the fluctuations in the amount of the compressed gas 136 due to the automatic control of the powder feed amount, and the fluctuations in powder discharge rate and in the pattern caused by the fluctuations cannot be neglected.
If the powder sticking to the inner surface of the duct has grown to an excessive degree, the operation of the apparatus is temporarily stopped, and the pinch valve 148 is closed. Then a large amount of gas is blown into the duct 140 through the injector nozzle 137 and the gas inlet 149 by some other means (not shown) in order to remove the powder from the inner surfaces of the feeding duct 140. After that, normal operation can be started again. Thus, the operation has to be interrupted to clean the duct. Further, since the sticking powder cannot be completely removed from the inner surface of the feeding duct 140 even by the above blowing operation, with the result that the powder feeding rate inevitably increases, though gradually. Thus, after the elapse of a fixed length of time, the feeding duct 140 has to be replaced by a new one.
In the above prior-art technique, the essential structure of which is shown in FIGS. 21 and 22, the powder tank, from which powder is introduced to the measurement duct 101, may contain a powder portion which is hard to fluidize. In order to fluidize such a powder portion, the tank itself or a porous plate provided therein is vibrated by some auxiliary means. In some cases, however, the fluidization cannot be effected even by vibrating the entire tank, making it impossible for the powder to be smoothly fed. Though vibrating the above-mentioned porous plate mostly proves effective, it has a problem in that the service life of the porous plate is shortened.
When a single tank is used with a number of powder feeding apparatuses operated in parallel, the tank must be large. The fluidization plate must be large as well. To operate such a large tank and large fluidization plate, a great quantity of fluidization air has to be consumed, resulting in a large cost.
When powder feeding is conducted by using a small amount of carrier gas, it is advantageous to extract powder from the bottom of the tank, as shown in FIG. 20. However, when cleaning the interior of the powder feeding system, it takes a long time to detach the system from the tank. Further, if it is detached after stopping the fluidization, some powder inevitably spills, thereby contaminating the environment.
Further, when applied to powder coating, the above system entails the following problem: when blowing is used to remove powder sticking to the inner surface of the feeding duct or to effect color change, powder is ejected at high speed from the tip of the gun, thereby contaminating the inner walls of the booth. It takes time and labor to clean the booth. It also takes time to clean the feeding duct portion which is inside the booth and to remove powder sticking to the exterior of the gun, closing the system down for a considerably long time for cleaning, color change, etc.
The measurement duct 101 and the injector throat 138 are usually made of a non-adhesive fluororesin or a high-density polyethylene. Despite using such materials, some powder may solidify in the interior of these components, thereby impairing the system functions. In the above prior-art technique, it is impossible to cope with such a situation when powder is to be fed with a very small amount of carrier gas and, in particular, when the measurement and feeding of powder is to be conducted with the powder flow-rate detection gas only.